Lab Report on Chemical Kinetics (Initial Rates Method & Activation Energy from the Temperature Dependence of the Reaction Rate)

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The lab report below was submitted as part of the coursework for CM1131 Basic Physical Chemistry. Please do not plagiarise from it as plagiarism might land you into trouble with your university. Do note that my report is well-circulated online and many of my juniors have received soft copies of it. Hence, please exercise prudence while referring to it and, if necessary, cite this webpage.
1. Aim: To determine the reaction orders and rate constant of a chemical reaction, using the method of initial reaction rates as well as to determine the activation energy from the temperature dependence of the reaction rate based on Arrhenius’ theory.

2. Results & Calculations:
2.1 Determination of Reaction Orders and Rate Constant
Molarity of KI:                   0.2000M
Molarity of S2O82- :           0.1000M
Molarity of S2O32-:            0.003300M
Solution
Vol. S2O82-
(mL)
Vol.
I-
(mL)
Vol.
H2O
(mL)
Vol. Starch
(mL)
Vol. S2O32-
(mL)
Time
(s)
Time
(s)
Average Time
(s)
1
10
10
0
1
5
20
20
20
2
10
8
2
1
5
24
24
24
3
10
6
4
1
5
35
36
35.5
4
10
5
5
1
5
46
45
45.5
5
10
3
7
1
5
77
82
79.5









6
10
10
0
1
5
20
20
20
7
8
10
2
1
5
25
25
25
8
6
10
4
1
5
35
36
35.5
9
5
10
5
1
5
41
39
40
10
3
10
7
1
5
86
83
84.5
Q1. Total volume of solution in the conical flask for each reaction is 26mL = 26cm-3
In Solution 1 to 5:             [S2O82-]                 = (0.01 × 0.1) ÷ 0.026                       = 0.03846 moldm-3
In Solution 1:                      [I-]                          = (0.01 × 0.2) ÷ 0.026                       = 0.07692 moldm-3
In Solution 2:                      [I-]                          = (0.008 × 0.2) ÷ 0.026                    = 0.06154 moldm-3
In Solution 3:                      [I-]                          = (0.006 × 0.2) ÷ 0.026                    = 0.04615 moldm-3
In Solution 4:                      [I-]                          = (0.005 × 0.2) ÷ 0.026                    = 0.03846 moldm-3
In Solution 5:                      [I-]                          = (0.003 × 0.2) ÷ 0.026                    = 0.02308 moldm-3

In Solution 6 to 10:           [I-]                          = (0.01 × 0.2) ÷ 0.026                       = 0.07692 moldm-3
In Solution 6:                      [S2O82-]                 = (0.01 × 0.1) ÷ 0.026                       = 0.03846 moldm-3
In Solution 7:                      [S2O82-]                 = (0.008 × 0.1) ÷ 0.026                    = 0.03077 moldm-3
In Solution 8:                      [S2O82-]                 = (0.006 × 0.1) ÷ 0.026                    = 0.02308 moldm-3
In Solution 9:                      [S2O82-]                 = (0.005 × 0.1) ÷ 0.026                    = 0.01923 moldm-3
In Solution 10:                   [S2O82-]                 = (0.003 × 0.1) ÷ 0.026                    = 0.01154 moldm-3

Solution
[S2O82-]
(moldm-3)
[I-]
(moldm-3)
Time
(s)
Time
(s)
Average Time (s)
ln[S2O82-]

ln[I-]

1
0.03846
0.07692
20
20
20
-3.258
-2.565
2
0.03846
0.06154
24
24
24
-3.258
-2.788
3
0.03846
0.04615
35
36
35.5
-3.258
-3.076
4
0.03846
0.03846
46
45
45.5
-3.258
-3.258
5
0.03846
0.02308
77
82
79.5
-3.258
-2.565








6
0.03846
0.07692
20
20
20
-3.258
-2.565
7
0.03077
0.07692
25
25
25
-3.481
-2.565
8
0.02308
0.07692
35
36
35.5
-3.769
-2.565
9
0.01923
0.07692
41
39
40
-3.951
-2.565
10
0.01154
0.07692
86
83
84.5
-4.462
-2.565

Q2. Reaction between I2 and S2O32-:                        I2+2S2O32-            2I-+  S4O62-
No. of moles of S2O32- reacted   =(5 x 10-3) × 0.003300      = 1.650 × 10-5mol
Since I2 ≡ 2S2O32-, no. of moles of I2 reacted = ½(1.650 × 10-5) = 8.250× 10-6mol
Therefore, no. of moles of I2 reacted/L= 8.250× 10-6 ÷ 0.026 = 3.173 × 10-4 molL-1

Rate of reaction of:
Solution 1            = 3.173 × 10-4 molL-1 ÷ 20s                             = 1.587 × 10-5 molL-1s-1
Solution 2            = 3.173 × 10-4 molL-1 ÷ 24s                             = 1.322 × 10-5 molL-1s-1
Solution 3            = 3.173 × 10-4 molL-1 ÷ 35.5s                         = 8.938 × 10-6 molL-1s-1
Solution 4            = 3.173 × 10-4 molL-1 ÷ 45.5s                         = 6.973 × 10-6 molL-1s-1
Solution 5            = 3.173 × 10-4 molL-1 ÷ 79.5s                         = 3.991 × 10-6 molL-1s-1
Solution 6            = 3.173 × 10-4 molL-1 ÷ 20s                             = 1.587 × 10-5 molL-1s-1
Solution 7            = 3.173 × 10-4 molL-1 ÷ 25s                             = 1.269 × 10-5 molL-1s-1
Solution 8            = 3.173 × 10-4 molL-1 ÷ 35.5s                         = 8.938 × 10-6 molL-1s-1
Solution 9            = 3.173 × 10-4 molL-1 ÷ 40s                             = 7.932 × 10-6 molL-1s-1
Solution 10          = 3.173 × 10-4 molL-1 ÷ 84.5s                         = 3.755 × 10-6 molL-1s-1

Q3.
Solution
[I-]
molL-1
ln[I-]
molL-1
ln[S2O82-]
Rate (R)
molL-1s-1
ln R
1
0.07692
-2.565
0.03846
-3.258
1.587 × 10-5
-11.05
2
0.06154
-2.788
0.03846
-3.258
1.322 × 10-5
-11.23
3
0.04615
-3.076
0.03846
-3.258
8.938 × 10-6
-11.63
4
0.03846
-3.258
0.03846
-3.258
6.973 × 10-6
-11.87
5
0.02308
-3.769
0.03846
-3.258
3.991 × 10-6
-12.43







6
0.07692
-2.565
0.03846
-3.258
1.587 × 10-5
-11.05
7
0.07692
-2.565
0.03077
-3.481
1.269 × 10-5
-11.27
8
0.07692
-2.565
0.02308
-3.769
8.938 × 10-6
-11.63
9
0.07692
-2.565
0.01923
-3.951
7.932 × 10-6
-11.74
10
0.07692
-2.565
0.01154
-4.462
3.755 × 10-6
-12.49
Table 2.1.2: Values for the logs of [I-], [S2O82-] and the rate R.


Q4. Gradient of best-fit-line (n) is 1.1781.

 

From the graph, the equation of best fit line is y=1.1781x-8.0004 where y = ln R and x = ln[I-]. The gradient of the graph is 1.1781 and thus, reaction order with respect to [I-], n=1.1781≈1 (to nearest integer).


Q5. Gradient of best-fit-line (m) is 1.1861.
 

From the graph, the equation of best fit line is y=1.1861x-7.1477 where y = ln R and x = ln[S2O82-]. The gradient of the graph is 1.1861 and thus, reaction order with respect to ln[S2O82-],n=1.1861≈1 (to nearest integer).

Q6.
Since n = 1.216 ≈ 1 and m = 1.247 ≈ 1 (nearest integer),

Since n = 1.216 ≈ 1 and m = 1.247 ≈ 1 (nearest integer)
Solution
[I-] molL-1
[S2O82-] molL-1
Rate (R) molL-1s-1
k
1
0.07692
0.03846
1.587 × 10-5
0.005364
2
0.06154
0.03846
1.322 × 10-5
0.005586
3
0.04615
0.03846
8.938 × 10-6
0.005036
4
0.03846
0.03846
6.973 × 10-6
0.004714
5
0.02308
0.03846
3.991 × 10-6
0.004496




6
0.07692
0.03846
1.587 × 10-5
0.005364
7
0.07692
0.03077
1.269 × 10-5
0.005362
8
0.07692
0.02308
8.938 × 10-6
0.005035
9
0.07692
0.01923
7.932 × 10-6
0.005362
10
0.07692
0.01154
3.755 × 10-6
0.004230
Table 2.1.3: Determined values for m, n and k.

Q7. Average value of k  = = 5.055 × 10-3 mol-1Ls-1

Standard deviation,                = 4.437 × 10-4mol-1Ls-1
2.2 Temperature Effect on a Chemical Reaction

Temp (0C)
Temp (K)
1/T (K-1)
Time (s)
Time (s)
Average Time (s)
ln(t)
1
59.0
332.15
0.003011
11
10
11
2.398
2
44.0
317.15
0.003153
20
18
19
2.944
3
31.0
304.15
0.003288
76
75
76
4.331
4
20.5
293.65
0.003405
338
-
338
5.823
5
8.0
281.15
0.003557
1242
-
1242
7.124
Table 2.2.1: Average time taken for the mixture of solution to turn blue at different temperatures. Temperature was kept constant for each experiment.

Graph 2.2.1: ln t against 1/T with temperature kept constant for each experiment

Since EA/R = 9142.5K,
Therefore EA         = 9142.5K × 8.314 JK-1mol-1
                                = 76010 Jmol-1
                                = 76.01 KJmol-1
Gradient of graph (i.e. EA/R) is 9142.5

3. Discussion:
3.1 Determination of Reaction Orders and Rate Constant
The experiments are conducted based on the rate equation, R = k [I-]n[S2O82-]m, where k is the rate constant while n and m are the reaction orders of I- and S2O82- respectively. As reaction orders, n and m is defined as the power to which the concentration of that reactant is raised to in the experimentally determined rate equation.nand mcannot be found theoretically and are experimentally determined to be 1. This means that the reaction is first order with respect to [I-] and first order with respect to [S2O82-]. The overall rate order is 2.This reaction is said to be bimolecular since two reactant species are involved in the rate determining step.

It was observed that the rate of reaction increases with increasing concentration. The Collision Theory explains the phenomenon by stating that for a chemical reaction to occur, reactant molecules must collide together in the proper orientation and the colliding molecules must possess a minimum energy known as the activation energy, EA, before products are formed. An increase in the concentration of reactants leads to an increase in the number of reactant molecules having energy ≥ EA, hence increasing the collision frequency. The increase in the effective collision frequency leads to an increase in the reaction rate.

When performing a chemical kinetics experiment, the procedures have to be conducted at a constant temperature. According to the Arrhenius equation,
k=Ae-Ea/RT, a slight increase in temperature increases reaction rate significantly as the equation is exponential in nature. This is affirmed by the Maxwell-Boltzmann distribution curve (diagram on the right) as a slight increase in temperature increases the number of colliding particles with Ea and consequently, reaction rates, significantly.
Hence, because slight deviations in temperature may affect reaction rates significantly, the temperature at which the experiment was carried out must be kept constant.


To prevent errors from occurring, all glassware used in this experiment must be kept clean and dry to prevent contamination by the previous batch of experimental products. The overall volume of the solution was also kept constant at 26mL by adding deionized water, to standardize the conditions of the reaction environment, thus increasing accuracy.

Swirling of the conical flask contents for the same length of time must be done consistently so that results obtained will be fair. Instead of swirling with one’s hands, the conical flasks can be placed on an electronic swirl to ensure consistent swirling when conducting the experiment.

Also, there is inaccuracy as the stopwatch was stopped only when an arbitrary colour intensity was observed. There should be a consensus between lab partners as to when the stopwatch should be stopped.
3.2 Temperature Effect on a Chemical Reaction
The results of this set of experiment show that the rate of reaction increases as temperature increases. Using the Arrhenius equation, k=Ae-Ea/RT, the activation energy, EA, can be determined by keeping the concentration of all the reactants constant while varying the temperature for each experiment.

When performing a chemical kinetics experiment, the procedures have to be conducted at a constant temperature. According to the Arrhenius equation,
k=Ae-Ea/RT, a slight deviation in temperature changes reaction rate significantly. This is affirmed by the Maxwell-Boltzmann distribution curve (diagram on the right) as a slight increase in temperature increases the number of colliding particles with Eaand consequently, reaction rates, significantly.
Hence, since slight deviations in temperature may affect reaction   rates significantly, the temperature at which the experiment was carried out must be kept constant.


This is especially important for experiments being conducted at 10oC and 20oC, the conical flasks were placed in an ice bath to maintain the reaction temperature. There were several fluctuations above and below the desired temperatures. Moreover, the time taken for the blue solution to turn colourless is relatively longer for these 2 lower temperatures which creates a greater room for error. Keeping temperatures constant can be done by conducting the experiments in a thermostatic water bath.

Reactants were poured imprecisely into the conical flask. There may be leftover reactants in the test tubes and some reactants may stain the sides of the conical flask during the addition. This reduces the concentration of the reactants in the conical flask. Pipetting the reactants into the conical flask would ensure that the reactants are added in the requisite quantities and that the eventual results are accurate.

Swirling of the conical flask contents for the same length of time must be done consistently so that results obtained will be fair. Instead of swirling with one’s hands, the conical flasks can be placed on an electronic swirl to ensure consistent swirling when conducting the experiment.

Also, there is inaccuracy as the stopwatch was stopped only when an arbitrary colour intensity was observed. There should be a consensus between lab partners as to when the stopwatch should be stopped.

The reaction is autocatalysed as the product of the reaction acts as a catalyst for the reaction. An autocatalysed reaction is slow at first and then becomes more rapidly as the catalyst is produced in the reaction. For the reaction, Mn2+ is the autocatalyst. This accounts for why vigorous effervescence of CO2 is not observed immediately when the reactants were added but only observed after a little while when Mn2+ is produced.

2MnO42- + 5C2O42- + 16H+ -> 2Mn2++10 CO2 + 8H2O


4. Conclusion:
The rate equation of the chemical reaction between I- and S2O82- to produce I2 and SO42- has been found to be:
 Rate = k[I-][S2O82-],        where rate constant k =5.055 × 10-3 mol-1Ls-1
The reaction is first order with respect to [I-] and the reaction is first order with respect to [S2O82-]. The overall order of reaction is 2. This reaction is said to be bimolecular since two reactant species are involved in the rate determining step.

Using the Arrhenius equation, k=Ae-Ea/RT, the activation energy, EA, of the oxidation reaction of oxalic acid by permanganate was determined to be 76.01KJmol-1. This means that the minimum amount of energy that reactant particles must possess in order to react successfully is experimentally determined to be 76.01KJmol-1.

5. References:
3)http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/AUTOCAT/PAGE1.HTM

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